1. Field of the Invention
The present invention relates to a best corrected visual acuity characteristics measuring device and a best corrected visual acuity characteristics measuring method suitable for use in measuring visual acuity characteristics of an eye with best correction.
The present invention also relates to a contrast sensitivity measuring device and a contrast sensitivity measuring method suitable for precisely determining the contrast sensitivity of a subject at the time of a refractive surgery or improvement of visual acuity with glasses or contact lenses.
The present invention further relates to a contrast target displaying device which can measure contrast sensitivity of an eye to be examined with best correction.
2. Description of Prior Art
A contrast sensitivity measuring device is used to precisely determine the effect of the ocular aberration of a subject on its visual function at the time of a refractive surgery or improvement of visual acuity with glasses or contact lenses. The measurement of contrast sensitivity is conducted on the premise that the corrective value obtained in a frame test (lens exchange method) is a value for best correction. In case of the lens change method, it is based on subjective vision test.
However, in a frame test, an eye is regarded as being best corrected when it obtains a visual acuity of 1.5 or higher in decimal visual acuity since the subject can obtain corrected visual acuity necessary for daily life. For example, there are many cases where a subject having an eye which needs −4.5 diopters for best correction can obtain a decimal visual acuity of 1.5 with a −3.75 diopter correction in a frame test. Thus, in a frame test, a value on the under-correction side may be regarded as the value for best correction. The “best correction” is a state where an eye to be examined focuses at infinity without adjustment.
In a clinical site, contrast sensitivity is used to distinguish eyes with lesions such as ametropia, cataract or keratoconus from normal eyes. When an eye is under-corrected, the maximum value of its luminance contrast sensitivity may not be obtained in a contrast sensitivity test. Namely, in the case where the visual acuity corrected based on a test result by a lens exchange method is not the best corrected visual acuity but an under-corrected visual acuity, when the eye is measured for contrast sensitivity wearing a frame spectacles with a corrective value obtained by the test, the eye may be estimated to have abnormally low contrast sensitivity. Then, a clinical technologist or ophthalmologist may judge there is abnormality in, for example, the visual nerve system behind the retina. As a result, a retest is conducted on a normal eye which has normal contrast sensitivity when best corrected on the premise that the eye has a lesion, resulting in increase in medical cost due to double inspection.
The “best correction” herein includes a slightly over-corrected state as well as the ideal best corrected state because an eye can obtain the maximum visual acuity value by adjustment when slightly over-corrected. When under-corrected, an eye can never obtain a visual acuity which is higher than the corrected value since an image blurs.